fix(database): DataSnapshot is always present (firebase#6754)

* fix(database): DataSnapshot is always present

* feat(database): DataSnapshot.exists

* test(database): DataSnapshot.exists tests

* format: run formatter

* test(database): update unit test mock

Author: russellwheatley

packages/firebase_database/firebase_database/example/test_driver/firebase_database_e2e.dart

@@ -100,10 +100,28 @@ void testsMain() {
           .child('ordered/one')
           .get();
       expect(dataSnapshot, isNot(null));
-      expect(dataSnapshot!.key, 'one');
+      expect(dataSnapshot.key, 'one');
       expect(dataSnapshot.value['ref'], 'one');
       expect(dataSnapshot.value['value'], 23);
     });
+
+    test('DataSnapshot.exists is false for no data', () async {
+      final dataSnapshot = await FirebaseDatabase.instance
+          .reference()
+          .child('a-non-existing-reference')
+          .get();
+
+      expect(dataSnapshot.exists, false);
+    });
+
+    test('DataSnapshot.exists is true for existing data', () async {
+      final dataSnapshot = await FirebaseDatabase.instance
+          .reference()
+          .child('ordered/one')
+          .get();
+
+      expect(dataSnapshot.exists, true);
+    });
   });
 }
 

packages/firebase_database/firebase_database/lib/src/event.dart

@@ -29,7 +29,7 @@ class Event {
 /// A DataSnapshot contains data from a Firebase Database location.
 /// Any time you read Firebase data, you receive the data as a DataSnapshot.
 class DataSnapshot {
-  DataSnapshot._(this.key, this.value);
+  DataSnapshot._(this.key, this.value) : exists = value != null;
 
   factory DataSnapshot._fromJson(
     Map<Object?, Object?> _data,
@@ -55,6 +55,9 @@ class DataSnapshot {
 
   /// Returns the contents of this data snapshot as native types.
   final dynamic value;
+
+  /// Ascertains whether the value exists at the Firebase Database location.
+  final bool exists;
 }
 
 class MutableData {

packages/firebase_database/firebase_database/lib/src/query.dart

@@ -82,7 +82,7 @@ class Query {
   Future<DataSnapshot> once() async => (await onValue.first).snapshot;
 
   /// Gets the most up-to-date result for this query.
-  Future<DataSnapshot?> get() async {
+  Future<DataSnapshot> get() async {
     final result = await _database._channel.invokeMethod<Map<dynamic, dynamic>>(
       'Query#get',
       <String, dynamic>{

packages/firebase_database/firebase_database/test/firebase_list_test.dart

@@ -330,14 +330,17 @@ class MockEvent implements Event {
 }
 
 class MockDataSnapshot implements DataSnapshot {
-  MockDataSnapshot(this.key, this.value);
+  MockDataSnapshot(this.key, this.value) : exists = value != null;
 
   @override
   final String key;
 
   @override
   final dynamic value;
 
+  @override
+  final bool exists;
+
   @override
   // ignore: no_runtimetype_tostring
   String toString() => '$runtimeType[$key, $value]';